The invention relates to an arrangement for determining the phase position of a data signal in the form of digital sampling values, comprising asynchronous modulated data having a known modulation frequency.
A data signal whose bits or data are modulated asynchronously but whose modulation frequency is known may have different phase positions which are to be determined. The data signal is present in the form of digital sampling values. Particularly when the sampling frequency at which the digital sampling values are gained is not coupled to the modulation frequency of the data, it is not known which sampling value represents which bit and where this sampling value is located within this bit. Considerable problems then occur when decoding the bits, or the decoding may be liable to disturbances, for example, noise.
To solve this problem, a signal-separating device is known from EP-A-0 472 756 which is used for separating a teletext signal from a video signal. The circuit operates in a very elaborate way and includes an integrating device which supplies a half-position signal characterizing the signal means of the individual bits of the data signal, and a residual value signal. A residual value signal evaluation device evaluates each residual value and determines, by means of a selection control signal, how the value of the current signal bit is to be evaluated. To this end, a selection device is available which forms the respective leading mean value or the trailing mean value from each current sampling value and each previous or subsequent sampling value and, by means of the selection control signal, selects the value suitable for the evaluation. A further phase correction device evaluates a signal sample supplied by a slope logic device, indicating whether the current sampling value approximates a slope of a signal. The same device then controls a change of the integration period when the residual value signal is within a predetermined residual value signal range. This arrangement operates in a very elaborate manner and can only perform a kind of estimation of the position of the individual bits.
It is an object of the invention to provide an arrangement of the type described in the opening paragraph, which is capable of determining the phase position of the individual bits, solves the above-mentioned problems, has a possibly simple construction and may at least partly be realized also in software.
According to the invention, this object is solved in that the arrangement comprises means performing a first and/or a second method of determining the bit positions of the sampled data signal, in that in the first method at least a set of three or five consecutive sampling values is searched whose mean sampling value is larger than or smaller than the neighboring outer sampling values of the set and in which the difference(s) of those outer sampling values being equidistantly spaced apart from the central sampling value fall below a predetermined threshold value, while upon detection of such a data set the position of the central sampling value and an associated position number indicating the position of the bits in the data signal are stored in a memory, in that in the second method a set of four consecutive sampling values is searched whose two central sampling values are approximately equally large and whose central value is smaller or larger than the central value of the two outer sampling values and in which the difference of the outer sampling values falls below a predetermined threshold value, while upon detection of such a data set the bit position in the center between the two central sampling values and an associated position number are stored in a memory (10), and in that the arrangement determines a phase signal from at least two determined bit positions, the position numbers assigned thereto and the period length of the bits of the data signal, which phase signal supplies the phase position of the bits of the sampled data signal relative to a predeterminable starting point.
In the invention, the phase position of the individual bits of the asynchronous modulated data are determined, while a subsequent decoder, which does not form part of the invention and which determines the data, receives a phase signal indicating, to the decoder, the phase position of the asynchronous modulated data in the data signal.
This problem may even be greater in that in the case of an unfixed coupling of the sampling frequency at which the digital sampling values of the data signal are generated and the modulation frequency at which the data in the data signal are modulated the phase position is not fixed or is not known. Then there is a drift between the individual sampling values and the position of the bits in the data signal. It is thus not known which sampling value represents which data bit or where this sampling value is located within the signal variation of this bit.
It is exactly this problem that the invention should solve and supply a phase signal indicating to a subsequent decoder the phase position of the modulated bits of the data signal relative to a predeterminable starting point.
To be able to supply such a phase signal, the arrangement according to the invention must be capable of determining the exact position of the bits in the data signal. For this purpose, the arrangement comprises means which perform a first and/or a second method in which the bit positions of the sampled data signals are determined. In this method, given bit positions having a marked position relative to the sampling values are selected, for example, within a predeterminable time interval or data interval of the data signal, so that the position of the bits can be concluded relatively precisely from the sampling values.
In the first method, these means select a set of three or five consecutive sampling values, for which the following conditions apply. The central sampling value of these three or five or more sampling values must be larger or smaller than all of its neighboring outer sampling values, i.e. than the other two or four sampling values of this set of sampling values. Furthermore, the difference between the outer sampling values, each being equidistantly spaced apart from the central sampling value, must be minimal. Thus, when three sampling values in one set are used in this method, the difference between these two values must be minimal. When five sampling values in one set are used for performing this method, the difference between the first and the last sampling value of this set of sampling values must be minimal and, furthermore, the difference between the second and the fourth sampling value of this set must also be minimal.
When a set of consecutive sampling values is found, for which these conditions have been fulfilled, it can be assumed that the central sampling value of this set of sampling values quite exactly represents a maximum or a minimum in the signal variation of the data signal. This sampling value thereby fairly exactly represents a central position of a bit in the data signal. Since exactly this bit is searched, the position of the central sampling value is stored in a memory when detecting such a data set for which the conditions described above have been fulfilled. Furthermore, the position of this bit in the data signal is stored, i.e., for example, which bit as from a predeterminable starting point is concerned. Also this value is stored as a position number in the memory.
After performing the first method, for example, for a given period of time or a given section of the data signal, a plurality of bit positions is found for which the conditions described above have been fulfilled and whose position numbers are then stored in the memory.
Additionally or alternatively to the first method, the means perform a second method in which a set of four consecutive sampling values is searched, in which the difference between the two central sampling values falls below a predetermined threshold value and in which these two sampling values are both smaller or both larger than the two outer sampling values of this set. Furthermore, the difference between the outer sampling values may have to fall below a predetermined threshold value. When such a data set is found, it can be concluded that the center of the bits to which this data set belongs fairly exactly indicates a position between the positions of the two central sampling values. A bit position in the center between these two sampling values and again the associated position number indicating the position of the bit in the data signal is thus stored in a memory. Here again, it holds that during performance of the method a plurality of such data sets complying with the above-mentioned conditions is found.
The arrangement now determines the phase signal from the stored bit positions and position numbers. This is possible because the length of the bits, i.e. their period length, is also known by virtue of the known modulation frequency. At least two of the stored determined bit positions are used, while it is known which positions these bit have and which one of the bits is concerned. It is also known, for example, for two stored bit positions by how many bits these positions are spaced apart and which positions the centers of these bits have. When the distance between the centers of the bits, the positions of the centers of the bits and the period length are known, the phase position can thus be directly determined from these two known values. By evaluating the quality of the individual measuring points, the exactness of the phase position to be determined can be enhanced. The arrangement is thus capable of indicating the phase position of the data signal or the phase position of the bits of this sampled data signal relative to a predeterminable starting point.
For a subsequent decoder, which is not a subject of the invention, it is thus simply possible to decode the bits because the positions of the centers of the bits in the data signal is exactly known to the subsequent decoder by virtue of the phase signal. The decoder can thus directly determine the positions of the sampling values relative to these centers of the bits so that it can perform an optimal decoding operation.
For determining the phase signal, at least two of the determined bit positions are to be used but the exactness of the method may be further enhanced in that more than two of the bit positions found are used for determining the phase signal.
The arrangement operates quite simply because only differences are to be formed for performing the method and further estimations are not necessary. Using relatively simple computations, the bit positions and also the phase signal can be determined. The functions performed by the arrangement are completely or partly realizable also in software.
An embodiment of the invention as defined in claim 2 ensures that the two methods of determining the positions of the bits in the data signal are used for the synchronizing bits in the data signal which precede, for example, a frame of data bits. The phase signal may then be gained from these synchronizing bits, which phase signal may be used for data bits transmitted after the synchronizing bits in the data signal. After transmission of the synchronizing bits, the phase position of the data signal for the next data block is known and a possibly subsequent decoder may receive an optimal phase signal for decoding the data bits.
In a further embodiment of the invention as defined in claim 3, only one of the bit positions found and stored in accordance with the first and the second method is used for generating the phase signal, using a minimal number of components. When easily distinguishable phase positions result from the stored bit positions, a mean value is formed between them and supplied as a phase signal. A quite precise phase signal can thus still be generated, while using a minimal number of components.
When position numbers assigned to a plurality of bit positions are determined and stored in the memory in the two methods, it is possible, in accordance with a further embodiment as defined in claim 5, to advantageously make a qualitative selection therefrom. This may be advantageously effected in that those sets of sampling values are used for which the differences do not only fall below the predetermined threshold values but are also minimal. This means that those data sets are used whose above-described differences are smallest. It is highly probable that those data sets are then found which best indicate the exact central position of each bit in the data signal.
As described hereinbefore, the arrangement can compute the phase signal from the stored bit positions, the associated position numbers and the known modulation frequency. Conversely, as in an embodiment of the invention as defined in claim 6, the modulation frequency of the data signal can be determined from the distances between the determined bit positions and the associated position numbers. This information may be used, for example, for identifying the data signal, i.e. so as to test whether the modulation frequency corresponds to the expected modulation frequency, or to determine by means of the modulation frequency which type of data signal is concerned. When the data signal is a teletext signal of a video signal, this information may be used, for example, for determining the transmission standard of the teletext signal.
In a further embodiment of the invention, as defined in claim 7, an error signal may be advantageously triggered in the arrangement when not at least one predetermined number of bit numbers is determined within a predetermined time interval. Then it can be assumed that it is either not the expected data signal or a disturbed data signal.
In a further embodiment as defined in claim 8, the arrangement may be advantageously used for determining the phase position of teletext signals within a television signal. The position of the teletext bits within a picture line of the television signal may fluctuate. To nevertheless enable a subsequent decoder to perform a reliable decoding operation, the arrangement according to the invention may advantageously supply a phase signal for each picture line.
In a further embodiment of the invention as defined in claim 10, the slope detector may be employed in the methods described above only for those sampling values that are associated with pulses having at least a predetermined slope height. It can thereby be achieved in that single interference peaks within the data signal are not recognized as bit position. Furthermore, as defined in this claim, a subsequent bit counter is controlled by means of the output signal of the slope detector, which bit counter directly supplies the position numbers of the individual bits and whose value can be directly used as a position number to be stored when carrying out the methods.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.